The present invention relates to thermo-engraving machines for printing in relief.
Thermo-engraving or typography in relief is a known process, which makes it possible to obtain from typographic printing, offset or the like, an impression in relief which is an imitation of copper plate printing or stamping.
In the known process, a freshly-printed sheet of paper is dusted or deposited with a powder having the property of melting under heat, and after cooling, forming a film in relief. Only the moist ink retains the powder, and the excess is continuously recovered. After the printed material is deposited with powder, then cleaned of excess, and printed material passes under a tunnel oven for heating. At the outlet of the oven, a jet of cool air cools the printed material and instantaneously solidifies the viscous film in relief so as to prevent the printed material from sticking together.
One powder used provides a glossy or mat finish and is transparent to preserve the tints of the printing colors. On the other hand, irrespective of the printing color, pigmented powders will give a relief corresponding to their pigment. The granule size of the powder employed determines the thickness of the film in relief. The coarser the powder granules, the greater the relief.
The prior art machines limit the possibilities of this process because of several inadequacies and defects. One disadvantage is that the tunnel ovens of these machines always operate at the full width or format, and in the majority of cases, the printed sheets to be treated are of varying widths. As a result, it is common practice to pass printed sheets of 10 cm (centimeters) in width into an oven that would permit the passage of sheets of 35 cm or more. This results in a disproportionate and useless waste of heat and powder.
Also, these machines only work with a single size of powder granules at the same time. Heavy printing characters may necessitate a coarse powder to give a relief of sufficient thickness, while on the contrary, thin characters may require fine powders in order to preserve their fineness in definition. Furthermore, certain portions of the printed text may be desired to be without relief, while others are desired to be in relief or of one or more different colors. These treatments require several passes through the machine, which greatly increases the costs of the treatment.
In the case of packing paper, wallpapers or labels having relief printing, it would be an advantage if the film forming the relief possessed a certain mechanical strength, which is not at present the case. The resins employed are usually polyamides with a low molecular weight, which are not subjected to any secondary action capable of improving their strengths.
Another disadvantage is the floor space taken up by these machines. Because the printing machines normally work at a high output, on the order of 4,000 to 10,000 copies per hour, an adequate length of tunnel ovens and cooling conveyors is required, especially when treating cards of substantial thickness.
These machines do not generally have automatic control of all of their adjustments in respect to the formats, paper weight and frequency of the successive sheets to be treated. Their users are obligated to regulate separately, often in an arbitrary manner, the control such as the speed travel of the conveyors and the temperature of the ovens.
In one type of machine, powder is deposited by gravity and the surplus powder is obtained by suction through a cyclone device. Rotating circular knives in the suction nozzle prevent the paper from being drawn up into the cyclone device. The contact of the knives or disks on the paper reduces the inlet air at the knife at the point of contact. This may result in traces of powder along the whole length of the paper. This is particularly troublesome in the case of colored powders. This defect also occurs in another form, on the back of the printed sheet. The powder conveyor is in fact in contact with the whole surface of the sheet, and has in spite of the suction, a tendency to fail to recover all of the granules of powder on the conveyor, whatever precautions may be taken.
The thermo-engraving machines necessitate frequent cleaning to remain in correct operation, since they deal with large quantities of powder per hour, of which only a small portion remains with the printed material. Cleaning operations are long and difficult at the present time because the components are not readily accessible and removable. In the case of mechanical breakdown or accident, these machines require a long and difficult dismantling operations, which result in loss of time.
During its passage through the interior of the oven, the paper is severely dehydrated. This results in a loss of firmness, which is recovered only more or less after a long period, depending upon the moisture content of the ambient air.
In the case of an accident, the operator must simultaneously stop the conveyor and the powder device, and must rapidly raise the tunnel oven. Otherwise, the paper stopped underneath is liable to catch on fire at once.
The collection of relief-printed sheets presents problems, since the edge of the sheet received has a tendency to catch on the parts in relief of the preceding sheet.
Another disadvantage is that the tray receptacle on prior art machines is located at one point, while the printing press is located at a fair distance away. This compels the operator of the printing press and thermo-engraving machine to move constantly to and fro while watching the two machines.